ci: specify exact ruby version

Otherwise bundler may complain if ruby-setup's 2.7.x differs from what's
in our Gemfile.

Author: peff

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     - name: ruby setup
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7
+        ruby-version: 2.7.2
         bundler-cache: true
 
     - name: rails setup